1. Field of the Invention
The present invention relates to inhibiting corrosion in gas treating solutions comprised of alkanolamine solutions or other solvents used in the removal of hydrogen sulfide, carbon dioxide, mercaptans or other acid gases from natural gas or other hydrocarbon gases or liquids. Specifically, the present invention relates to passivating the metals in contact with the corrosive solutions by reducing the metal's oxidation state to a lower number. The reduced oxidation state results in a less corrosive, harder, impervious, and insoluble layer in contact with the treating solution. Additionally, the corrosion inhibitor may contain a metal oxide that will help to catalyze or increase the activity of the corrosion inhibitor and to also add passivation to pre-existing pits, crevices, or imperfections in the metal in contact with the gas treating solution.
2. General Background of the Invention
Contaminants in crude hydrocarbons subjected to refining or purification operations include acids or acid-forming materials such as CO.sub.2, H.sub.2 S, mercaptans, and sulfides. These acid-forming materials must be removed from the natural and cracked hydrocarbon or refined streams (which contain such hydrocarbons as methane, ethane, propane, etc. and olefins such as ethylene, propylene, butylene, etc). One typically used method of removing the acids and acid-forming materials from hydrocarbon gases or liquids is by absorption in an amine regenerative solution absorbent unit. Regenerative amine solution units include columns with trays or other packing which are used to contact the aqueous alkanolamine solution with the hydrocarbon gases or liquids which contain the acids or acid-forming compounds. The amine solution can be regenerated by thermal stripping with steam to remove the acids or acid-forming compounds such as H.sub.2 S, CO.sub.2, mercaptans and sulfides. This is accomplished in a regeneration section of the unit comprised of a column with trays or other packing in which the amine is contacted with steam, a reboiler in which the steam is formed, a reflux condenser and return system in which the steam is conserved, and other associated heat exchange equipment used for energy conservation or subsequent cooling of the amine prior to its return to the absorption section of the unit. Due to the presence of these acids and acid-forming compounds, corrosion is often observed in the equipment containing the solutions.
The metallurgy of the equipment contacting the treating solution is usually carbon steel or stainless steel. The iron in these steels are typically hydrolyzed or oxidized to any of the following iron hydroxides or iron oxides: Fe(OH)2, Fe(OH)3, FeO(OH), Fe2O3, or Fe3O4. The latter of these, Fe3O4 or magnetite, is the hardest, most impervious, and most insoluble of the iron oxides or iron hydroxides. Due to the much lower corrosion potential, it is highly desirable to maximize the conversion of iron in contact with the treating solution to the magnetite form.
Corrosion rates in the equipment sustaining the treating solution increase with increased amine concentration and acid gas concentration in solution. This usually limits the overall capacity of the treating solution for removal of more acid gas components from the gas or liquid stream it contacts. Corrosion results because the stability of the hydrolyzed or oxidized form of the steel that generally provides some passive resistance to corrosion is reduced when amine or treating solution concentration increases and when the concentration of the acid component in solution with the treating solution increases. By strengthening the passive film, the system capacity for handling more acid gas removal per unit volume of treating solution can be increased.